Flat memory model

Flat memory model or linear memory model refers to a memory addressing paradigm in which "memory appears to the program as a single contiguous address space."[1] The CPU can directly (and linearly) address all of the available memory locations without having to resort to any sort of memory segmentation or paging schemes.

Memory management and address translation can still be implemented on top of a flat memory model in order to facilitate the operating system's functionality, resource protection, multitasking or to increase the memory capacity beyond the limits imposed by the processor's physical address space, but the key feature of a flat memory model is that the entire memory space is linear, sequential and contiguous from address zero to MaxBytes − 1.

In a simple controller, or in a single tasking embedded application, where memory management is not needed nor desirable, the flat memory model is the most appropriate, because it provides the simplest interface from the programmer's point of view, with direct access to all memory locations and minimum design complexity.

In a general purpose computer system, which requires multitasking, resource allocation and protection, the flat memory system must be augmented by some memory management scheme, which is typically implemented through a combination of dedicated hardware (inside or outside the CPU) and software built into the operating system. The flat memory model (at the physical addressing level) still provides the greatest flexibility for implementing this type of memory management.

The majority of processor architectures implement a flat memory design, including all early 8-bit processors, the Motorola 68K series, etc. One exception to this rule was the original 8086, Intel's first 16-bit microprocessor, which implemented a segmented memory model because it allowed access to more than 64 KiB of memory without the cost of extending all addresses to more than 16-bits.

Comparison

Most modern memory models fall into one of three categories:

Flat memory model

Paged memory model

X86 Segmented memory model

Main article: Intel Memory Model

Within the x86 architectures, when operating in the real (compatible) mode, physical address is computed as:[2]

Address = 16 × segment + offset

(i.e. The 16-bit segment register is shifted left by 4 bits and added to a 16-bit offset, resulting in a 20-bit address.)

Von Neumann vs. Harvard architectures

Von Neumann architecture

A Von Neumann architecture specifies that instructions and data are both stored within the same memory and transferred over the same bus. This type of architecture is the most space efficient, because any memory not used by the program is available for data and vice versa. It is most often used in general computing. Additionally, a program can treat its own instructions as data and modify them during execution. This is known as self-modifying code.

Harvard architecture

Main article: Harvard architecture

A Harvard architecture separates instructions and data into two separate memories which are typically accessed over two separate buses. The benefit is an increase in system performance because data (for the present instruction) and code (for the future instruction) can be fetched at the same time. Also, in a Harvard architecture, the instruction and data buses can have different speeds and geometries. The drawback is that Harvard architectures force the designer to make assumptions about the ratio of instruction vs data memory, and this memory cannot be re-allocated at run time. The system is therefore unavoidably more wasteful. This type of architecture is most often used in digital signal processing (DSP), real-time control and ultra high-speed RISC applications, where performance is of utmost importance and everything else is secondary.

Neither of the above architectures determines whether the memory addressing model is flat, segmented, paged or otherwise.

See also

References

  1. ↑ Gonzalez, Antonio; Latorre, Fernando; Magklis, Grigorios (2011). Processor Microarchitecture: An Implementation Perspective. Morgan & Claypool Publishers. p. 72. ISBN 9781608454525.
  2. ↑ General description of Real Mode “The physical address can be calculated as Value_in_segment_register
    • 16 + Value_in_offset_register.”
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